1. Field of the Invention
The present invention relates to a structure of a field emission type electron emitting device using a semiconductor fine processing technique and relates to a method of producing the same.
2. Description of the Related Art
In recent years, micro vacuum electronic tubes have been produced so as to be applied to display units, high-speed switching devices, various kinds of sensors, etc. Hereupon, a technique of forming a micro electron source skillfully has become a key technology. Heretofore, a hot cathode type electron emitting device using thermoelectrons emitted from a heated filament, or the like, has been used popularly as an electron source. The hot cathode type electron emitting device, however, has problems in the large loss of energy caused by heating, the necessity of preparatory heating, etc. To solve these problems, public attention has been paid onto a field emission type (cold cathode type) electron emitting device, and some proposals have been made.
FIG. 11 is a partly perspective view showing an example of the field emission type electron emitting device. This is now called "conical (or pyramidal) electron emitting device 101". As shown in the drawing, a conical emitter 12 made of molybdenum (hereinafter simply referred to as "Mo") or the like is provided on a silicon substrate 11. There is formed an insulating layer 14 of a silicon oxide, or the like, having a portion opened around the emitter 12. Further, a gate electrode 13 having an end portion formed in the vicinity of the pointed end portion of the conical emitter 12 is provided thereon. In the field emission type electron emitting device configured as described above, when a voltage is applied between the silicon substrate 11 and the gate electrode 13, electrons are emitted from the pointed end portion of the emitter 12 which is high in the intensity of electric field.
FIGS. 13A to 13E are partly sectional views in respective steps for explaining the method of producing the conical electron emitting device 101 shown in FIG. 11. The steps will be described below with reference to the drawings.
An insulating layer 14 is formed on a silicon substrate 11. Further, by an electron beam vapor deposition method, the insulating layer 14 is coated with an Mo layer 131 which constitutes a gate electrode 13. Then, a photoresist is applied thereonto and then subjected to exposure and development so that a first pattern 161 is formed as shown in FIG. 13A. Then, the Mo layer 131 and the insulating layer 14 are selectively etched with use of the photoresist pattern 161 as a mask to thereby form a first opening portion 181 and a second opening portion 182. The Mo layer 131 having the first opening portion 181 is formed as a gate electrode 13 as shown in FIG. 13B. Then, the silicon substrate 11 is inclined by a predetermined angle .theta. while rotated in a substrate plane, so that aluminum (hereinafter abbreviated to Al) is evaporated so as to be deposited on an upper face of the gate electrode 13 and on a side face of the first opening portion 181 to thereby form an Al layer 191 as shown in FIG. 13C. Then, by an electron beam vapor deposition method, Mo is applied perpendicularly to the silicon substrate 11. In this occasion, Mo is deposited not only both on the upper face of the Al layer 191 and on the silicon substrate 11 but also on the side face of the Al layer 191. Accordingly, the diameter of the first opening portion 181 decreases as the Mo layer 192 is deposited. Because the vapor deposition range of Mo deposited on the silicon substrate 11 decreases as the diameter of the first opening portion 181 gradually decreases, a nearly conical emitter 12 is formed on the silicon substrate 11 as shown in FIG. 13D. Finally, the deposited Mo and Al layers 192 and 191 are removed to thereby form a conical electron emitting device 101 having such a nearly conical emitter 12 as shown in FIG. 13E.
In the field emission type electron emitting device of FIG. 11 according to the aforementioned producing method, there is however such a tendency that reproducibility in the case where the same shape is repeatedly formed is not satisfied because the conical emitter 12 is formed by vapor deposition. For this reason, there arises a disadvantage in that electron emitting characteristic particularly sensitively influenced by the radius of curvature of the topmost end of the emitter 12 and by the distance between the emitter 12 and the gate electrode 13 varies widely.
Upon such a background, an electron emitting device having a new shape and being good in uniformity of electron emitting characteristic has been published recently in Journal of Semiconductor World, March 1992, p. 62, by Kanamaru and Ito. FIG. 12 is a partly perspective view of the electron emitting device. This is now called "a comb-like electron emitting device 102". An insulating layer convex portion 241 and an insulating layer concave portion 242 are formed in an insulating layer 24 on a silicon substrate 21 (FIGS. 14A-14D). An emitter 22 made of Mo and having a plurality of emitter end portions 221 on one side is disposed on the insulating layer convex portion 241. On the other hand, a gate electrode 23 is formed on the insulating layer concave portion 242 so as to be opposite to the emitter end portion 221. Also in this electron emitting device, by applying a voltage between the emitter 22 and the gate electrode 23, electrons are emitted from the end of the emitter end portion 221 which is high in the intensity of electric field. This structure can be produced relatively easily by a conventional semiconductor producing process, so that this producing method is a method considerably improved in reduction of scattering in the producing steps. Further, not only the emitter 22 and the gate electrode 23 shown in FIG. 12 can be formed but also other electrodes such as an anode electrode for collecting emitted electrons, a control electrode for controlling electrons reaching the anode electrode, and so on, can be formed.
FIGS. 14A to 14D and FIGS. 15A to 15C are partly sectional views showing the steps of producing the comb-like electron emitting device 102 shown in FIG. 12. The steps will be described below successively. For example, an oxide film as an insulating layer 24 is applied onto a silicon substrate 21. Further, a tungsten film (hereinafter simply referred to as "a W film") 222 which constitutes an emitter is deposited on the whole surface of the insulating film 24 by means of sputtering (FIG. 14A). Then, a photoresist is applied onto the W film 222 so that a first pattern 261 is formed by using a photomask not shown. The W film 222 is etched by reactive ion etching (RIE) with use of the photoresist pattern 261 as a mask (FIG. 14B). Further, the insulating layer 24 is etched by about 1 .mu.m with use of the resist pattern 261 and the W film 222 as a mask so that an insulating layer convex portion 241 and an insulating layer concave portion 242 are formed (FIG. 14C). A niobium film (hereinafter abbreviated to "an Nb film") 231 which constitutes a gate electrode 23, and an aluminum film (hereinafter abbreviated to "an Al film")/Mo film 232 are applied onto the substrate by vacuum vapor deposition. The Al film/Mo film 232 and the Nb film 231 on the insulating layer convex portion 241 are removed by a lift-off method (FIG. 14D). A photoresist is applied again so that a second pattern 262 is formed by using a second mask not shown. The Al film/Mo film 232 and the Nb film 231 are etched by reactive ion etching (RIE) with use of the photoresist pattern 262 as a mask (FIG. 15A). Further, a photoresist is applied once more so that a comb-like pattern 263 is formed by using a third mask not shown. By reactive ion etching (RIE) with use of the photoresist pattern 263 as a mask, a comb-like emitter 22 is formed. In this occasion, the gate electrode 23 is not masked but the AL film serves as a protection film so that the gate electrode 23 is not processed into a comb shape (FIG. 15B). Finally, the Al film/Mo film 232 is etched and the surface of the insulating layer 24 is further etched with a buffer hydrofluoric acid, so that electrical insulation between the emitter and the gate electrode is improved. Thus, this process is completed (FIG. 15C). As metal materials used for the emitter and the gate electrode, W, Mo, Nb, etc. are selected on the basis of work function expressing the degree of easiness of flying of electrons, surface stability in the process and after the process, durability in a long term, etc.
As FIGS. 14A to 14D and FIGS. 15A to 15C show the producing steps, not only the comb-like electron emitting device 102 of FIG. 12 is large in the number of photoetching steps, that is, large in the number of times for forming a photoresist pattern and for performing etching with use of the photoresist pattern as a mask, but also many kinds of metal materials are used in the comb-like electron emitting device 102 of FIG. 12 compared with the structure of the conical electron emitting device 101 shown in FIG. 11. Accordingly, there is a limitation in selection of the etching method and etching solution to be used. Further, the emitter is produced from a polycrystalline metal thin film. In the thin film, there is some crystal grain boundary between crystals having a size of about 0.1 .mu.m. Because there is difference in etching speed, for example, in dry etching, between the inside and the outside with respect to the grain boundary, the shape of the end portion of the emitter is apt to be formed along the grain boundary. There arises a disadvantage in that as a result, the shape varies widely. Because the shape of the end portion of the emitter has a direct influence onto a field emission current emitted from the emitter, it is difficult to practically use the emitter the shape of which varies widely.
As described above, the conventional field emission type electron emitting device is not sufficient for the design concerning the combination of structures and materials. As a result, not only it is a matter of course that electron emitting characteristic varies correspondingly to each lot, but also the characteristic is not uniform in the same substrate in the same lot. Further, in the producing process, a measure to solve the problem caused by the defect in the combination of structures and materials is not taken, either.